Method for extrusion

ABSTRACT

Die arrangement for use with conventional and hydrostatic extrusion presses consisting of a first die for providing a first extruded section of a billet and a second die cooperating with the first die to further extrude said first extruded section to final shape. Successive dies beyond the second die are contemplated to provide successive step-wise extrusion for greater reductions, especially of large billets. The method of the invention comprises step-wise incremental sequential extrusion of the billet and extruded portions of the billet until the final shape is achieved.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my U.S. Patent applicationSer. No. 642,907 filed Dec. 22, 1975, now U.S. Patent 3,999,415.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of extrusion and, inparticular, to those extrusion processes wherein the object beingextruded, e.g. a billet of metal or other extrudable material, is forcedthrough a die by mechanical or hydrostatic means. In particular, theinvention pertains to those extrusion processes where successiveextrusions are accomplished on the original billet in a step-wisefashion. More particularly, the invention pertains to multiple diearrangements in order to provide greater extrusion reductions on a givenextrusion press or apparatus wherein a solid or hollow billet undergoesmultiple reductions without increasing the extrusion pressure.

2. Description of the Prior Art

Extrusion processes have been used for many years for producingsemi-finished shapes in metals such as bars, wire, tubing, andcomplicated finished shapes such as H's, angles, and the like.Conventional extrusion processes are employed for both hot and coldextrusion, e.g. where the billet undergoing extrusion is either raisedto an elevated temperature or is extruded at ambient, the former beingmore common for metals and the latter for plastics. The use of anelevated temperature will generally depend upon the material beingextruded, the size of the initial billet, and the size and shape beingextruded. Both hot and cold processes also encompass the use oflubricants and other aids to minimize the friction between the die andthe material being extruded. Conventional extrusion processes areillustrated in U.S. Pat. Nos. 2,123,416 and 2,135,193. Extrusion diesused in such processes, and in particular in a multiple die set areillustrated in U.S. Pat. No. 3,553,996.

More recently, the hydrostatic extrusion technique has become widelyadopted for use in extruding materials heretofore difficult to extrudeby techniques illustrated by the above patents or materials that,because of their propensity to oxidize at elevated temperature ormaterials that require closer dimensional control, are better suited tocold extrusion. Generally, cold extrusion of such materials byconventional techniques requires equipment capable of very highextrusion pressures. In the hydrostatic technique, a fluid raised to anelevated pressure forces the billet through the die to achieve the finalshape. An excellent discussion of the history of hydrostatic extrusionis contained in the specification of U.S. Pat. No. 3,491,565.Hydrostatic extrusion processes are illustrated in U.S. Pat. Nos.3,126,096; 3,343,388; 3,677,049; and 3,893,320. One type of hydrostaticextrusion die is illustrated in U.S. Pat. No. 3,583,204.

In addition, materials can be and have been formed into elongated shapesby drawing through a die. Such processes are illustrated in U.S. Pat.No. 3,740,990.

It is well known in the art that the maximum allowable reduction for abillet undergoing an extrusion process is limited by the extrusionpressure applied to the billet as it enters the extrusion die, thebillet material flow stress, and the die friction. In both hot and coldextrusion, whether conventional or hydrostatic, the limits of allowablestresses in the components of the extrusion apparatus (extrusionchamber, ram, and dies) restrict the maximum extrusion pressure that canbe produced with a given apparatus. Thus, regardless of the design ofthe extrusion apparatus, the maximum extrusion pressure and resultantreductions accomplished by that pressure are limited by the materials ofconstruction of the apparatus.

The reduction limit means that, for many extruded products, the initialbillet must be limited in cross-sectional area, otherwise the reductionwill have to be accomplished in successive steps. This size limitationis most severe in conventional and hydrostatic extrusion processes thatare carried out at ambient temperature (so-called cold extrusionprocesses) because of the high flow stress and work hardening of thematerial being extruded. The advantage of large extrusion reductions,associated with hot extrusions, must be sacrificed if the improvementsin tolerances and properties resulting from cold extrusion are desiredor necessary. As pointed out above, cold extrusion may be the onlyprocess available if the material is one that oxidizes readily, orsuffers some other form of degradation, at elevated temperature.

It is well known that hydrostatic extrusion processes are advantageousin that: (1) high pressures can be applied to the billet for greaterreduction ratios; (2) there is generally a small die cone angle; (3) theextruded product can be made to close dimensional tolerances; (4)conditions of good lubrication exist; and (5) there is less die wear.However, hydrostatic extrusion may not be available for use with someproducts because the extruded product volume (primarily determined bythe long length required) necssitates an initial billet, which is toolarge in diameter to be extruded in one step. When hydrostaticallyextruding certain materials, it becomes necessary to extrude thematerial into a pressurized container to increase the hydrostatic stressstate during the forming operation. This process requires a pressurizedcontainer of sufficient size to accept the entire extrusion productwhich limits the pressure differential across the extrusion die, thusfurther restricting the reduction ratio between the initial billet andthe extrusion product.

One problem often associated with hydrostatic extrusion is the unstable,so-called stick-slip extrusion action, which is usually minimized oreliminated by some form of mechanical action such as pushing on thebillet or pulling on the extrusion. This unstable extrusion action isessentially an unsolved problem when large reductions are attempted oncold or elevated temperature (below crystalline melting temperature)polymeric materials using the hydrostatic extrusion process.

SUMMARY OF THE INVENTION

The present invention pertains to a die assembly for use withconventional and hydrostatic extrusion presses wherein the die assemblyconsists of a first die which yields as a product a first extrudedportion of a billet forced through the first die. A second die in theassembly cooperates with the first die to extrude the first extrudedportion of the billet to final dimensions. With an assembly according tothe present invention, it is possible to take successive reductions byadding additional dies and the means to operate these dies, thusavoiding all extrusion reduction limitations normally associated withconventional and hydrostatic extrusion processes.

The method of the present invention comprises incremental sequentialextrusion of the original billet until the final size and shape areachieved. The method does not continuously extrude the billet in theusual sense, rather portions of the billet are extruded sequentially toachieve the final product. Using the method of the present inventionhelps to minimize and, in a large number of cases, eliminate thestick-slip associated with prior art hydrostatic extrusion processes.

Therefore, it is the primary object of the present invention to providean improve extrusion process.

It is another object of the present invention to provide an improved dieassembly and means for operating the dies for use with conventional andhydrostatic extrusion presses.

It is still another object of the present invention to provide animproved method and apparatus for extrusion which can overcome reductionlimitations of conventional apparatus by taking multiple reductions on abillet as it exits from an extrusion chamber with a multiple diearrangement.

It is yet another object of the present invention to provide anextrusion process that can be a combination hydrostatic and conventionaldie method.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1a, 1b, and 1c are fragmentary sectional views illustrating boththe method and apparatus of the present invention.

FIG. 2 is a fragmentary view partially in cross-section of a three-stagehydrostatic extrusion die assembly according to the present invention.

FIGS. 3a, 3b, and 3c are fragmentary cross-sectional views illustratingthe pressure balancing die seal assembly of the apparatus of FIG. 2.

FIG. 4 is a fragmentary view partially in section of a combinedhydrostatic and conventional extrusion process according to the presentinvention.

FIG. 5 is a fragmentary view partially in section of the apparatus ofFIG. 4 converted to a first and second stage hydrostatic extrusionapparatus.

FIG. 6 is a fragmentary view partially in section of a conventionalfirst-stage extrusion die in combination with a conventionalsecond-stage extrusion die employing the method of the instantinvention.

FIG. 7 is a fragmentary view partially in section illustrating a methodand apparatus for extruding elongated filamentary material according tothe present invention.

FIG. 8 is a fragmentary view partially in section illustrating a methodand apparatus for extruding tubular products according to the presentinvention wherein the inside diameter of the extrusion is constant ineach stage.

FIG. 9 is a fragmentary view partially in section illustrating a methodand apparatus for extruding tubular products according to the presentinvention wherein the inside diameter of the extrusion is reducedbetween stages.

FIG. 10 is a fragmentary view partially in section illustrating a methodand apparatus for extruding tubular products wherein a multi-componentmandrel is used.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, and in particular to FIGS. 1a, 1b, and 1c,there is shown a cylinder 10 having a first section 12 defining a first,or billet chamber 14. Billet chamber 14 terminates in a die opening 16.Below the die opening 16 is a second section 18 of cylinder 10 defininga receiving or first extrusion product chamber 20 for receiving theextrusion product. Slidably mounted within the opening 20 is a seconddie or die assembly shown generally as 22 having a die section 24 and apiston section 26. The bottom of cylinder 10 is closed by a suitableplug 28 which defines the one limit of the stroke or path of travelpermitted for second die 22. Of course, the upper limit of travel of thedie 22 is determined by the bottom or outlet of die opening 16. Inreferring to a die in the present specification, Applicant means thatstructure having an inlet opening, a smaller outlet opening, and adeformation zone therebetween as is well known in the art. The presentspecification is structured so that the dies are vertically orientedwith each successive stage below the previous one. The secondary die 22has a central bore 25 communicating with die opening 30. Die opening 30is placed so that it is immediately adjacent to die opening 16 and inaxial alignment therewith.

Conventional sealing members such as "O"-rings 32, 34, 36, and 38 areprovided in suitable grooves or recesses on the secondary die 22 and endclosure 28 respectively. Cylinder 10 includes conduits or ports 40, 42,and 44 respectively, the function of which will be explained in detailhereinafter. For illustrative purposes, a reservoir 46, conduits 48, 50,52 and check valve 56 and valve 54 are shown associated with conduits 42and 44. With the apparatus illustrated in FIGS. 1a, 1b, and 1c, it ispossible to accomplish extrusions in the following manner. Chamber 14 ofcylinder 10 defines a high pressure chamber which can be of extendedlength and closed at a location remote from die opening 16 by astationary closure (not shown) or moveable ram (not shown) to providethe driving force for the primary extrusion as is well known in the art.Chamber 14 is designed to withstand high operating fluid pressure sothat the original billet 60 can be extruded to a reduced section 62which section size is defined by the die opening 16. The fluidmaintained in chamber 14 is pressurized by a ram or external pump as iswell known in the art to cause the primary extrusion to proceed at apreselected rate. During this portion of the extrusion cycle, valve 54is closed, thus maintaining the fluid pressure in chamber 14. As theoriginal billet 60 is extruded through the primary die opening 16, thefirst or primary extrusion product 62 pushes on secondary die 22 causingit to move in a longitudinal direction. As shown in FIG. 1b, the actionof billet section 62 causes the die 22 and piston section 26 to cause afluid (normally disposed in the annulus defined by piston 26, endclosure 28, and the bottom portion of die 22 as illustrated in FIG. 1a,when die 22 is in contact with die opening 16) to be forced throughconduit 40 to a reservoir (not shown). Simultaneously, fluid iswithdrawn from reservoir 46 through a check valve 56 and conduit 50through conduit 42 into the cavity defined by primary extrusion 62 andwall 20 of cylinder 10 when the die 22 is in the lowermost position asillustrated in FIG. 1b.

When die 22 completes its stroke by having piston section 26 contact endclosure 28, the initial cycle of the primary extrusion stops. At thispoint, valve 54 is opened so that the cavity defined by wall section 20,first extrusion 62 and the top of die 22 is pressurized with fluid fromchamber 14. At this point, fluid is forced through conduit 40, thuscausing die 22 to move upwardly against primary extrusion product 62,causing it to flow through die opening 30, thus becoming a seconaryextrusion product 64 (FIG. 1c). As secondary die 22 performs theextrusion process on primary extrusion 62, fluid is forced out throughconduit 42 through valve 54 through conduits 52 and 44 into cylinder 14,thus maintaining pressure equilibrium in the fluid contained in thechamber 14. The secondary extrusion continues until die 22 has traveledits full stroke as shown in FIG. 1c. At this point, valve 54 is closedand the hydraulic fluid below piston section 26 is depressurized byflowing through conduit 40 into an external reservoir (not shown) andthe extrusion through die opening 16 of the billet 60 resumes under theaction of the fluid contained in chamber 14. This action initiatesanother extrusion cycle which continues in accordance with the cycledescribed hereinabove. Successive extrusion cycles are repeated untilthe desired amount of original billet 60 is extruded to the final sizeand shape 64. The process can encompass either complete or partialextrusion of the original billet 60 as desired.

The unstable stick-slip extrusion action, often associated withhydrostatic extrusion, can be controlled by the apparatus as illustratedin FIGS. 1a, 1b, and 1c for the primary extrusion through die opening 16by controlling the rate at which hydraulic fluid leaves the annulus orsecondary pressure chamber 61, defined by piston section 26, closure 28and the wall of second section 18 of cylinder 10 (FIG. 1c), therebycontrolling the rate at which secondary die 22 allows the initialextrusion 62 to flow through the primary die opening 16. Stick-slipextrusion will be eliminated during the second stage extrusion ofprimary extrusion product 62 through die opening 30 by the inherentstability caused by the fluid pressure in chamber 14 and the fluidpressure above die opening 30 in the chamber defined by primaryextrusion 62, chamber 20, and die surface 20, being slightly higher thanthat required for the unrestricted hydrostatic extrusion of primaryextrusion 62 through die opening 10, such that the primary billet 60 isheld stationary against die opening 16. It is possible to achieveextrusion through primary die opening 16 against a desired die pressureby providing a restraining action against primary extrusion 62 with thesecondary die 22 by maintaining pressure in the fluid below pistonsection 26 and simultaneously controlling the pressure of the fluidabove die opening 30 with an external pressure system (not shown).

In FIG. 2, there is illustrated a three-stage apparatus of the presentinvention. FIG. 2 serves to illustrate that the number of successiveextrusions can be large, being limited only by the size of the apparatusand the material being extruded. The operation of the device of FIG. 2is similar to that described in connection with FIG. 1, except thatpressure equalizing die seals are employed to replace the externalconduits and valves used for fluid transfer in the apparatus of FIG. 1.The die seals are shown generally as 70 and 72 of FIG. 2 and they areshown in more complete detail in FIGS. 3a, 3b, and 3c. Only one of theseals is illustrated in FIGS. 3a, 3b, and 3c; the other, acting in anidentical manner. Referring now to FIG. 2, the apparatus includes acylinder 80 having contained therein a two-piece primary die having alower portion 82 and an upper portion 84 with a micro port therebetween,the port being illustrated by line 86. The upper die 82-84 is sealedwithin cylinder 80 by pressure equalizing die seal 70. Pressureequalizing die seal 70 communicates with a conduit 88, the purpose ofwhich will be explained more fully hereinafter. Disposed below primarydie 82, 84 is a secondary die 90 having associated with it a secondarypressure chamber, the secondary pressure chamber 91 being defined by thebottom of 82, the upper surface of secondary die 90 and the inner wallof cylinder 80, secondary die 90 and tertiary pressure chamber assembly92 being sealed to the cylinder 80 by pressure equalizing die seal 72.Slidably mounted within tertiary pressure chamber 92 is a tertiary die94 having a die opening 96 and piston section 98. The cylinder is closedby end closure 105. Tertiary pressure chamber 92 has associated ventconduits 102. Conduit 104 is included through enclosure 100 to enablefluid to be forced against piston section 98 of tertiary die 96. Theextrusion process for extruding a billet 110 proceeds as described inrelation to FIGS. 1a, 1b, and 1c, except that there are now primaryextrusion 112, secondary extrusion 114, and the tertiary or finalextrusion 116, all accomplished in sequential fashion as described inrelation to FIG. 1.

Referring to FIG. 3, and in particular FIG. 3a, pressure equalizing dieseal 70 is illustrated prior to pressurization of fluid contained in thechamber defined by primary die 82, 84 and cylinder 80 and pressureequalizing die seal 70; this chamber being referred to as 120.

The pressure equalizing die seal comprises in combination an "O"-ring122, a compressible elastomer column 124, miter rings 126, 128, and ventring 130. As fluid pressure in chamber 120 is increased to effect theprimary extrusion, "O"-ring 122 retains the fluid pressure in chamber120, although the elastomer column 124 is compressed (FIG. 3b). As shownin FIG. 3c, as the pressure in chamber 120 increases with thetermination of the primary extrusion caused by secondary pressurechamber 92 contacting end closure 105, the elastomer column 124 iscompressed further so that "O"-ring 122 moves past micro port 86 andfluid contained in chamber 120 passes through micro port 86 into thecavity defined by the primary extrusion 122, adjacent die section 82,secondary extrusion die 90, and secondary extrusion chamber 91. Vent 88is included to prevent fluid pressure from building up in the elastomercolumn cavity and thus negatively influencing the operation of thepressure equalizing seal. Vent ring 130 is included to aid in minimizingpressure buildup on the elastomer column cavity. Miter rings 126, 138,in cooperation with the design of the vent ring 130, insure that theelastomer column 124 and "O"-ring 122 do not extrude in the low pressurezone of cavity 120.

Referring back to FIGS. 2 and 3, the apparatus operates as follows. Asthe billet 110 is extruded through primary extrusion die 82, thesecondary extrusion die 90 begins to move together with tertiarypressure chamber assembly 92 until the secondary die 90 and tertiarypressure chamber 92 travel the full stroke. After this has taken place,fluid pressure in fluid chamber 120 is increased a small amount(approximately 5% to activate the primary die pressure equalizing seal70 and allow fluid to flow from chamber 120 through micro port 86 intothe cavity between primary extrusion 112 and primary die section 82 thenbetween secondary die 90 and secondary pressure chamber 91. Hydraulicpressure is then applied to the piston supporting secondary die 90causing secondary die 90 to move against primary extrusion 112 andextruded this section 112 through the secondary die 90 to form secondaryextrusion 114. Excess fluid in the cavity between primary extrusion 112an secondary pressure chamber 91 will flow back through micro port 86into chamber 120.

After the secondary die has gone its full stroke, the pressure inchamber 120 and surrounding primary extrusion 112 is raised againapproximately 5% to activate the secondary die pressure equalizing seal72 and allow fluid to flow into the cavity surrounding secondaryextrusion 114. When the pressure surrounding secondary extrusion 114 isin equilibrium with the remainder of the high pressure fluid system, thelow pressure hydraulic system is pressurized through conduit 104 andtertiary extrusion die 94 begins to form tertiary extruded section 116.Upon completion of the tertiary extrusion 116, the primary chamber fluidpressure is reduced to deactivate the pressure equalizing seals, thenthe secondary and tertiary low pressure hydraulic pressures are reducedto zero gauge pressure. At this point, primary extrusion will againcommence and initiate the next extrusion cycle. Extrusion cycles arecontinued until the desired amount of the original billet 110 isextruded to the final shape 116.

FIG. 4 shows a conventional cold extrusion die as the second stage inconjunction with a primary hydrostatic extrusion die according to thepresent invention. The modification illustrated in FIG. 4 simplifies theoverall apparatus and its operation; however, there is an attendantsacrifice in the lubrication advantage of hydrostatic extrusion, thusmaking it necessary to apply a suitable lubricant to the billet; thecomposition and quantity of the lubricant being determined by the billetmaterial and the operational extrusion fluid as is well known in theart. As before, there is a primary cylinder 130 defining a primaryextrusion or pressure chamber 132. Cylinder 130 is closed at one end bya first die 134, generally referred to a the primary die. The die 134 issealed to the cylinder 130 by conventional sealing means such as"O"-ring 136. Chamber 132 is designed to withstand operational fluidpressures so that billet 138 can be extruded through primary die 134 toprovide a first extrusion section 140. The operational fluid containedin chamber 132 can be pressurized directly by a ram disposed in thechamber 132 or by sealing the chamber and using a suitable externalpump. Billet 138 is extruded at an appropriate rate to provide theprimary extrusion 140 which moves secondary die 142 until the pistonportion 144 of die 142 reaches the end of its stroke as determined byclosure 148, disposed in the end of cylinder 130. The pressure in thefluid-contained chamber 132 is then increased by approximately 5% ormore as required to insure that the billet 138 remains in contact withprimary die 134 during the secondary extrusion. After the pressure isincreased, hydraulic fluid is forced through conduit 150 acting onpiston section 144 of die 142 starting secondary extrusion through die142 producing a product extrusion 152. Secondary extrusion continuesuntil the secondary die 142 travels its full stroke as determined bypiston section 144 contacting the bottom of die 134. At this point,hydraulic pressure below piston section 144 of die 142 is reduced tozero or a suitable pressure required for a back pressure extrusion andbillet 138 is again forced through primary die 132 by the fluidcontained in chamber 132, thus initiating the next step. As before, thecycle continues until the desired amount of billet 138 is extruded tofinal size and shape.

There is shown in FIG. 5 a modification of the apparatus of FIG. 4wherein the second stage of the device of FIG. 4 is converted to asecond stage hydrostatic extrusion by the addition of "O"-ring seal 159on secondary die 142. The apparatus of FIG. 5 functions in a similarmanner as the apparatus of FIG. 4 until the primary extrusion 10 iscompleted. At this point, the fluid pressure in chamber 132 is loweredso that, when hydraulic fluid is forced against piston 144 of secondarydie 142, the secondary extrusion of primary extrusion 140 through die142 does not occur. Instead, secondary die 142 moves the product 140 tothe position shown in FIG. 5 so that the complete length of primaryextrusion 140 is exposed to the fluid pressure contained in chamber 132.

With the hydraulic pressure on piston section 144, sufficient to holddie 142 in place, the pressure of fluid in chamber 132 is raised tocause the primary extrusion product 140 to hydrostatically extrudethrough die 142. When the billet 138 comes in contact with primary die134 and extrusion through secondary die 142 stops, the hydraulicpressure on piston 144 is relieved and extrusion of billet 138 throughprimary die 134 begins, thus starting another extrusion cycle.

An apparatus according to FIG. 4 was constructed and used to extrude analuminum alloy of the 1100-0 type. A billet 11.0 millimeters in diameterwas hydrostatically extruded through the primary die having a dieopening of 3.40 millimeters at a fluid pressure of 63.0 kg/mm². Theprimary extrusion measured 11.4 millimeters in length and was thenconventionally extruded to 1.0 millimeters in diameter, 78 millimeterslong, by the secondary die at 98.0 kg/mm² extrusion pressure. The cycleswere successfully repeated to take an overall billet to final productreduction with 99.2% reduction in area of the original billet.

With an apparatus and method according to the invention, it is possibleto either use hot or cold extrusion techniques in conjunction with thepresent invention. The temperature at which the billet is extruded willdepend on the material itself together with the reduction desired.

FIG. 6 illustrates the application of the method of the presentinvention to an apparatus using entirely conventional extrusion whichmay be carried out at ambient or elevated temperature. Billet 160 isplaced into a conventional, cylindrical extrusion chamber 161 and forcedthrough primary die 162 by ram 163. In a manner of operation similar tothat presented for hydrostatic extrusion, the desired portion of billet160 is extruded into primary extrusion product 164 which is in turnextruded through secondary extrusion die 165 to produce secondaryextrusion product 166. The secondary extrusion is produced by forcingthe secondary die 165 against the primary extrusion product 164 whileholding billet 160 stationary with pressure from ram 163. Thus,conventional two-stage extrusion dies can be used to practice the methodof the instant invention.

It would also be possible to apply the method of the present inventionto extruding long filamentary products, e.g. wire products. In theextrusion of wire products, it may be necessary to provide anintermediate looping chamber to accumulate the previously extrudedmaterial prior to the next stage. Such gathering and looping inhydrostatic extrusion of continuous wire is illustrated in FIG. 7 of thedrawing.

In the method and apparatus of FIG. 7, extruded filament 172 from aprior extrusion stage having die outlet 171 enters into a fluid-filledcylindrical chamber 170 and forms filament loop 173, shown in dottedlines, with the aid of guide pins 174 and 179. Then, die 175 is forcedinto chamber 170 causing the fluid therein to be pressurized to apressure which hydrostatically extrudes filament 172 through die 175,yielding a long, filamentlike extrusion product 178. The die 175 isforced into chamber 170 by the action of die ram 177 which is madefluidtight with the help of "O"-ring 176. The extrusion of filament 172continues until it is stretched taut across guide pins 174 and 179 (asshown) thereby eliminating loop 173. Simultaneously, fluid pressure inchamber 170 rises above the pressure required to extrude the filament172 through die 175 causing a slight tension in filament 172. Thispressure rise signals the end of this extrusion cycle and the die ram177 force on die 175 is reduced to zero. The die 175 moves to allow thevolume of extrusion chamber 170 to increase and to allow the fluid inchamber 170 to be depressurized. Next, a new filament loop 173 isextruded into chamber 170 to initiate the next cycle.

This invention also applies to the extrusion of products having a hollowcross-section including, inter alia, tubular shapes. Mandrels forcontrolling the interior dimensions of the hollow products are shown inFIGS. 8, 9, and 10. FIG. 8 illustrates a mandrel 182 which remainsstationary with respect to the primary die 181. Hollow billet 180 ishydrostatically extruded through die 181 with mandrel 182 controllingthe inside dimensions of the primary extrusion product 185. The mandrel182 is fixed in the apparatus so that it remains stationary with respectto die 181. Mandrel 182 consists of a cylindrical portion fitting insidehollow billet 180; this cylindrical portion of mandrel 182 terminates atan integral, conical section located inside the deformation zone of die181. Extending axially from the small end of the conical section is anintegral, cylindrical section which extends past the exit of thesecondary extrusion die 184. This cylindrical section controls theinside dimensions of the secondary extrusion product 186 as it exitsfrom secondary extrusion die 184.

FIG. 9 shows an extrusion arrangement identical to that of FIG. 8 exceptthat stationary mandrel 187 has been modified. Mandrel 187 consists of acylindrical section fitting inside the hollow billet 180 whichterminates in an integral, conical section as before. However, thecylindrical section extending from the small end of the conical sectionextends only slightly beyond the primary die 181 outlet before it isreduced in diameter. The reduced diameter section of the mandrel extendsthrough the hollow primary extrusion product 185 and through thesecondary extrusion die 184. The reduced diameter of the extension ofmandrel 187 results in a reduced inside diameter of secondary extrusionproduct 188 as it exits from secondary extrusion 184.

FIG. 10 show a two-component mandrel arrangement for a two-stageextrusion of a tubing cross-section using this invention. In thisexample, the basic process is conventional extrusion. Hollow billet 201is accepted into the primary extrusion chamber 200 and forced throughprimary die 206 by a hollow ram not shown. Controlling the insidedimensions of the primary deformation zone of billet 201 as it flowsthrough primary die 206 is the hollow, cylindrical primary mandrel 202,which remains stationary with respect to die 206. The solid cylindricalsection of the secondary mandrel 203 slides inside of the primaryextrusion mandrel 202 and controls the inside diameter of the primaryextrusion product 208 as it exits from the primary die 206 and duringthe secondary extrusion process of primary extrusion product 208 throughsecondary die 204. The secondary mandrel 203 is mechanically orhydraulically constrained to move in cooperation with the secondary die204 always maintaining the same relative position with respect tosecondary die 204. The conical section of secondary mandrel 203 and theshort cylinder extending from the small end of the conical sectioncontrols the inside dimensions of the primary extrusion product as itflows through die 204 and exits as the secondary extrusion product 205.

It is obvious that the die assembly and the method of the presentinvention can be embodied in various forms and movement of one dierelative to the other can be accomplished in numerous ways and invarying sequences without departing from the spirit and scope of thepresent invention.

Of course, the invention is not limited in any respect to materials ofconstruction, the materials of construction being selected on the basisof the material being extruded.

In all embodiments of the invention, the pistons, cylinders, dies, dieholders, rams and the like can be manufactured in multiple parts as isknown in the art. While the invention is illustrated with the diesvertically oriented, the orientation of the dies is not critical andthey may be operated in a horizontal, vertical, or acute angularposition.

Having thus described my invention, what I desire to have secured byLetters Patent of the United States is set forth in the followingclaims. I claim:
 1. A method for hydrostatic extrusion of a billetincluding the steps of:extruding a portion of the billet through a firstdie ths forming a primary extrusion section of the billet; stopping saidprimary extrusion followed by extruding the primary extrusion sectionthrough a secondary die, thus forming a secondary extrusion definingsaid desired size and shape of the finished extrusion;stopping saidsecondary extrusion which simultaneously initiating extrusion of thebillet to form a second primary extrustion section; and alternatelyextruding said billet and said primary extrusion sections until saidbillet is extruded to the desired corss-section dize, shape, and length.2. A method according to claim 1 wherein said secondary extrusion issubjected to an extrusion, thus forming a tertiary extrusion, beforesaid billet is subjected to further extrusion steps.
 3. A methodaccording to claim 1 wherein a plurality of incremental, ever decreasingin size extrusions are formed in a step-wise manner alternately untilsaid billet is extruded to the desired size and shape.
 4. A methodaccording to claim 1 wherein a hollow billet is extruded over a solidmandrel.
 5. A method according to claim 1 wherein said primary extrusionis an elongated filament which is accumulated in a pressure chamberwhich is pressurized after completion of the primary extrusion to forcethe primary extrusion through the secondary die.
 6. A method ofextruding a billet including the steps of:hydrostatically extruding aportion of the billet through a first die thus forming a primaryextrusion section of the billet; stopping said primary extrusionfollowed by extruding the primary extrusion section through a secondarydie by conventional extrusion means, thus forming a secondary extrusiondefining the desired size and shape of the finished extrusion; stoppingsaid secondary extrusion while simultaneously initiating extrusion ofthe billet to form a secondary primary extrusion section; andalternately extruding said billet and said primary extrusion sectionsuntil said billet is extruded to the desired cross-section size, shape,and length.
 7. A method according to claim 6 wherein said billet isfurther extruded by extruding said secondary extrusion by conventionalmeans to form a tertiary extrusion to a desired smaller cross-sectionsize, shape, and length.
 8. A method according to claim 6 wherein saidbillet is further extruded by conventional means to form a tertiaryextrusion section and said tertiary extrusion section is extruded byhydrostatic means to form a quarternary extrusion section.
 9. A methodof extruding a billet including the steps of:extruding a portion of thebillet through a first die by conventional means thus forming a primaryextrusion section of the billet; stopping said primary extrusionfollowed by hydrostatically extruding the primary extrusion sectionthrough a secondary die, thus forming a secondary extrusion defining thedesired size and shape of the finished extrusion; stopping saidsecondary extrusion while simultaneously initiating extrusion of thebillet to form a secondary primary extrusion section; and alternatelyextruding said billet and said primary extrusion sections until saidbillet is extruded to the desired cross-section size, shape, and length.10. A method of extruding a billet including the stepsof:hydrostatically extruding a portion of the billet through a first diethus forming a primary extrusion section of the billet; stopping saidprimary extrusion followed by hydrostatically extruding the primaryextrusion section through a secondary die thus forming a secondaryextrusion defining the desired size and shape of the finished extrusion;stopping said secondary extrusion while simultaneously initiatingextrusion of the billet to form a secondary primary extrusion section;and alternately extruding said billet and said primary extrusionsections until said billet is extruded to the desired cross-sectionsize, shape, and length.
 11. A method according to claim 10 wherein saidsecondary extrusion is hydrostatically extruded, thus forming a tertiaryextrusion before said billet is subjected to further extrusion steps.12. A method according to claim 10 wherein a plurality of incremental,ever decreasing in size extrusions are formed in a step-wise manneralternately until said billet is extruded to the desired size and shape.